Hand-held stereoscopic viewers have previously been developed for viewing stereo pairs of images arranged on viewing cards. Examples of such viewers include the following U.S. design patent Nos. 171,049; 174,902; 175,215; 184,356; 185,670; and No. 189,006. Other patents, which describe the mechanical action of such viewers, include U.S. Nos. 2,189,285 and 2,511,334.
Each of the foregoing patents discloses a hand-held viewer comprising a housing with a pair of spaced eyepieces through the front of the housing. The eyepieces each carry magnifying lenses through which the stereo images are viewed at the focal plane of the lenses. The rear of the housing contains a translucent area behind the film images aligned with the viewing axes of the lenses for introducing light into the housing and illuminating the stereo images. The translucent area is usually made of a milky plastic that obscures the internal parts of the housing. An advancing mechanism is provided in many of the housings for incrementally moving the card to display sequential images through the lenses. These advancing mechanisms typically comprise an integral advancing plate and a sidewardly extending finger actuated trigger lever. Actuation of the trigger moves the advancing plate within the housing to bring a pawl on the plate into engagement with a slot in the card to incrementally advance the displayed images.
The small, translucent windows which permit light to enter the housing are usually spaced a substantial distance from the viewing card such that movement of the card cannot be detected through the windows. The windows also occupy a limited area of the housing, thereby admitting a small amount of light into the interior of the housing for illumination of the film images.
The side-extending trigger of existing viewers can be awkward to operate since it is difficult to grasp the viewer and reciprocate the trigger with the same hand. The sidewardly extending trigger of the prior art must be moved along a vertical slot through the side of the housing from the top to the bottom of the viewer to move the actuator plate. Such a movement requires that the housing be firmly grasped by both hands since manipulation of the sidewardly extending trigger interferes with the trigger hand's hold on the housing.
The advancing plate which incrementally advances the card for viewing sequential images usually contains a flexible arm having a terminal pawl that engages a slot on the card to advance the card as the trigger moves the plate. The pawl will only work effectively if the card is firmly retained against the plate so that the pawl will be retained in the slot during advancement. In the prior art, the card was held against the plate by ridges or projections inside the housing which abutted the card when the card was in place on the plate.
It is accordingly an object of the invention to provide in a stereo viewer of the type described a rotatable advancing plate having means to engage and rotate a disc-like viewing card while simultaneously gripping the card and holding it flat against the plate.
It is another object of the present invention to provide a viewer which admits a greater amount of light to the housing interior for illumination of stereo images being viewed.
Another object of the invention is to provide such a viewer in which rotation of the viewing card and movement of at least some internal parts can be detected by an operator who actuates the trigger.
Even yet another object is to provide a trigger on the viewer which can be more comfortably actuated by a single finger while grasping the viewer with the same hand that actuates the trigger.